No abstract

Accelerated ageing based on floating has been used to investigate the state of health of high voltage carbon/carbon capacitors in aqueous Li 2 SO 4 electrolyte and to determine the factors influencing their life-time. A pressure sensor connected to the cell allowed to measure gas evolution caused by overcharging of the system and to determine the maximum operating voltage limit of the cell. At voltage higher than 1.5 V, gases started to evolve, together with oxidation of the positive carbon electrode material and formation of decomposition and/or corrosion products. The positive electrode was found to entail the ageing of the entire system, being covered by twice more surface oxygenated groups than the negative one after 120 h of floating at 1.7 V. The blockage of pore entrances by oxygenated surface functionalities decreases the accessible pore volume, causing a decrease of capacitance and an increase of internal resistance during floating. A drop of capacitive current is observed at voltage higher than 1.2 V on the voltammograms already after 20 h floating at 1.7 V. From the study, it can be concluded that the AC/AC supercapacitors using aqueous Li 2 SO 4 with stainless steel current collectors can operate safely up to 1.5 V, which is actually much more than the voltage value of 0.8-1 V reached with aqueous KOH and H 2 SO 4 electrolytes.

No abstract

Strategies are presented to enhance operating potential and cycle life of AC/AC capacitors using salt aqueous electrolytes. Li 2 SO 4 (pH = 6.5) allows 99% efficiency to be exhibited at 1.6 V cell potential with low self-discharge, while in BeSO 4 (pH = 2.1) efficiency is low (81%). Li 2 SO 4 performs better due to high di-hydrogen over-potential at the negative electrode and related pH increase in AC porosity. When stainless steel current collectors are used in Li 2 SO 4 , the cell resistance suddenly increases after 12 hours floating at 1.6 V, due to corrosion of the positive collector. With nickel negative and stainless steel positive collectors, the electrode potentials are shifted by −105 mV at cell potential of 1.6 V, allowing stable cell parameters (capacitance, resistance) and reduction of corrosion products formation on positive steel collector after 120 hours floating. Phenanthrenequinone was grafted on activated carbon to get an additional faradaic contribution in buffer solutions (pH = 4.0 or 7.2). The three-electrode cell CVs show that the redox peaks of the phenanthrenequinone graft shift toward negative values when pH increases from 4 to 7. Electrical double-layer capacitors (EDLCs) based on activated carbons (AC) and traditional aqueous electrolytes, such as H 2 SO 4 and KOH, operate at low cell potential (up to ∼0.8 V), limiting their capability at industrial level. [1][2][3][4] In order to improve the energy (E = 1 2 CU 2 ), both capacitance (C) and cell potential (U) determined by the stability window of the electrolyte have to be optimized. In this context, it has been recently demonstrated that symmetric capacitors based on AC electrodes and salt aqueous electrolyte (0.5 mol L −1 Na 2 SO 4 ) exhibit excellent cyclability under galvanostatic charge/discharge up to 1.6 V. 5,6 . While using 1 mol L −1 Li 2 SO 4 and gold current collectors, excellent cycle life has been shown up to ∼1.9 V using galvanostatic charge/discharge. 7,8 Such high cell potential value is caused by a high over-potential for di-hydrogen evolution as a consequence of water reduction and OH − ions generation in the porosity of the negative AC electrode. 5,7 According to the Nernst equation (E red = −0.059 pH), the pH increase associated to OH − ions causes locally a shift of redox potential to lower values. Based on this fact, it has been recently demonstrated that the di-hydrogen evolution over-potential is higher in almost neutral electrolyte solutions (pH = 4-8) than in acidic ones, resulting in larger operating cell potential of asymmetric capacitors in the former media.9 From the foregoing, it makes sense to study in more details the influence of aqueous electrolyte pH on the electrochemical performance of AC/AC capacitors beyond only the cell potential.Besides, when shifting from gold to stainless steel current collectors in order to develop low cost AC/AC capacitors in 1 mol L −1 Li 2 SO 4 , constant capacitance and low cell resistance have been observed during potentiostatic floating at cell potential of 1.5 V, whil...

No abstract